This invention generally relates to digital imaging apparatus and more particularly relates to a frame for and method for mounting polarization components and a reflective LCD spatial light modulator.
Initially introduced as small-scale imaging devices for business presentation markets, digital color projectors have steadily improved in overall imaging capability and light output capacity. In order for digital motion picture projectors to compete with conventional motion picture film projectors such as those used in theaters, however, a number of significant technical hurdles remain. Unlike conventional motion picture projectors, high-quality digital projection systems provide separate color modulation paths for red, green, and blue (RGB) color image data. The design of digital color projection apparatus requires that monochromatic light beams carrying images formed on each of the individual color channels be combined, with proper intensity and registration, in order to project a full color image.
Referring to FIG. 1, there is shown a simplified schematic for a digital motion picture projection apparatus 10 as described in U.S. patent application Ser. No. 10/050,309, incorporated herein by reference. Each color channel (r=Red, g=Green, b=Blue) uses similar components for forming a modulated light beam. Individual components within each path are labeled with an appended r, g, or b, appropriately. For the description that follows, however, distinctions between color channels are specified only when necessary. A light source 20 provides unmodulated light, which is conditioned by uniformizing optics 22 to provide a uniform illumination, directed through an illumination relay lens 80 to a dichroic separator 27. Dichroic separator 27 splits the white light into red, green, and blue color channels. Following any of the three color channels, light goes to a light modulation assembly 38 in which a relay lens 82 directs light through a prepolarizer 70 to a polarizing beamsplitter 24. Light having the desired polarization state is transmitted through polarizing beamsplitter 24 and is then modulated by a spatial light modulator 30, which selectively modulates the polarization state of the incident light over an array of pixel sites. The action of spatial light modulator 30 forms an image. The modulated light from this image, reflected from polarizing beamsplitter 24, is transmitted along an optical axis Or/Og/Ob through an analyzer 72 and is directed by a magnifying relay lens 28, through an optional folding mirror 31, to a dichroic combiner 26, typically an X-cube, Philips prism, or combination of dichroic surfaces in conventional systems. An optional color-selective polarization filter 60 may also be provided in the modulated light path. Dichroic combiner 26 combines the red, green, and blue modulated images from separate optical axes Or/Og/Ob to form a combined, multicolor image for a projection lens 32 along a common optical axis O for projection onto a display surface 40, such as a projection screen.
The reflective liquid crystal device (LCD) of FIG. 1 is a type of spatial light modulator that is widely used in digital projector design. This device accepts polarized light and modulates the polarization of the incident light to provide colored light beam as output. For obtaining polarized light, a polarizing beamsplitter prism, such as a McNcille prism, is typically employed along with the support of one or more polarizing elements, configured as polarizers and analyzers.
Because modulated light must be combined from each of three color channels in order to synthesize a color image, correct registration of the modulated light is important. When the modulated light is reflected from the surface of spatial light modulator 30, angular errors in the relative alignment of each LCD surface can cause significant shifts in resolution, yielding unsatisfactory image quality. Further image quality problems, such as loss of contrast, can be the result of imperfect alignment of polarization support components, particularly for polarizing beamsplitter 24. Moreover, thermal expansion effects can cause further drift in registration and degrade polarization components performance. Thermal expansion becomes a particular concern with high-end projection apparatus, since high brightness is required in these applications. At the same time, compact optical packaging is desirable, with minimized optical path length between image-forming components and the projection lens. These conflicting requirements complicate the design of high-brightness projection apparatus.
The negative impact of thermal expansion on image registration is well known in the art. In response to this problem, U.S. Pat. No. 6,345,895 (Maki et al.) discourages use of a mounting base for supporting reflective spatial light modulators, polarizing beamsplitters, and related polarization support components. Significantly, the U.S. Pat. No. 6,345,895 disclosure even teaches away from the use of a mounting base formed from metals or composite materials having low coefficients of expansion. Instead, the approach proposed U.S. Pat. No. 6,345,895 mounts spatial light modulator components directly to glass prism components used for beamsplitting or color combining, so that components in the optical path remain in alignment with thermal expansion. This same overall type of approach is also taught in U.S. Pat. No. 6,375,330 (Mihalakis); U.S. Pat. No. 6,053,616 (Fujimori et al.); and U.S. Pat. No. 6,056,407 (linuma et al.).
One recognized problem with attachment to prism components is in achieving the initial alignment itself. As one example, U.S. Pat. No. 6,406,151 (Fujimori) describes methods for adhesively affixing LCD components to a prism with alignment. While attachment directly to a glass or plastic prism surface may have advantages for minimizing thermal expansion effects, there appear to be a number of drawbacks with solutions that use adhesives, compounding thermal dissipation concerns for the LCD itself and making component replacement a costly and time-consuming procedure.
Recently, as is disclosed in U.S. Pat. No. 6,122,103 (Perkins et al.), high quality wire grid polarizers have been developed for use in the visible spectrum. While existing wire grid polarizers may not exhibit all of the necessary performance characteristics needed for obtaining the high contrast required for digital cinema projection, these devices have a number of advantages. Chief among these advantages are the following:
(i) Good thermal performance. Wire grid polarizers do not exhibit the thermal stress birefringence that is characteristic of glass-based polarization devices, as was noted above.
(ii) Robustness. Wire grid polarizers have been shown to be able to withstand anticipated light intensity, temperature, vibration, and other ambient conditions needed for digital cinema projection.
(iii) Good angular response. These devices effectively provide a higher numerical aperture than is available using conventional glass polarization beamsplitters, which allows relatively higher levels of light throughput when compared against conventional devices.
(iv) Good color response. These devices perform well under conditions of different color channels. It must be noted, however, that response within the blue light channel may require additional compensation.
U.S. Patent Nos. 6,234,634 and 6,447,120 (both to Hansen et al.) and U.S. Pat. No. 6,585,378 (Kurtz et al.) disclose image projection apparatus using wire grid polarizing beamsplitters. The wire grid polarizing beamsplitter offers advantages over conventional prism-based polarizing beamsplitters, particularly due to its small size and weight. It can be appreciated that there could be advantages for light modulation in a combination using wire grid polarizer and analyzer components. However, as with the more conventional beamsplitter and polarizers employed in prior art projection apparatus, wire grid components are themselves subject to thermal expansion effects and must be properly aligned with respect to the spatial light modulator within each color channel, with thermal effects taken into account.
An article in the SID 02 Digest entitled xe2x80x9cThe Mechanical-Optical Properties of Wire-Grid Type Polarizer in Projection Display Systemxe2x80x9d by G. H. Ho et al., presents some of the key design considerations for deploying wire grid polarizer components in imaging apparatus using reflective LCD spatial light modulators. Noting problems caused by mechanical constraint and thermal stress in a comparatively low-power projection apparatus, the Ho et al. article highlights the overall negative impact of conventional mounting techniques for wire grid polarizing beamsplitters. Notably, the Ho et al. disclosure is directed to an imaging system that uses a reflective LCD spatial light modulator that transmits modulated light thru a wire grid polarizing beamsplitter. Inherent problems in that type of system include astigmatism, which can be corrected using techniques described in the Ho et al. article. Among other problems noted in the Ho et al. article are surface deformation caused by thermal effects on the wire grid polarizing beamsplitter. It can be appreciated that problems for low-to intermediate-power projection apparatus, as highlighted in the Ho et al. article, would be even more pronounced for higher energy projection equipment.
Among key design considerations for mounting a wire grid polarizing beamsplitter is maintaining the surface of this component at an accurate. 45 degree orientation relative to both the surface of the spatial light modulator and the surface of an analyzer. A related problem that must be resolved in electronic projection apparatus design is alignment of the spatial light modulator itself relative both to the wire grid polarizing beamsplitter and to the projection optical path. Maintaining precision alignment without the negative effects of thermal drift is a key design goal for high-end electronic projection apparatus.
Unlike the imaging application of the Ho et al. configuration, projection apparatus 10 of FIG. 1 (of which the present invention is part) uses reflective LCD spatial light modulator""s 30r, 30g, 30b that direct modulated light back to the corresponding polarizing beamsplitter""s 24r, 24g, 24b, which in turn reflect light towards the imaging lens. In order to substitute wire grid polarizing beamsplitter""s for conventional prism based polarizing beamsplitter components, the thermal effects highlighted by Ho et al. must be considered. However, because the position of the polarizing beamsplitter is as a reflective surface in the path of modulated light, the inherent thermal impact on imaging problems is even more pronounced than for the system described in the Ho et al. article. That is, with wire grid polarizing beamsplitter components used in place of polarizing beamsplitter""s 24r, 24g, 24b, convergence, contrast, and general wave front aberrations are serious concerns for the optical designer. These optical effects are due to surface deformation, lateral shifts, or tilt and/or rotations, and all of which can be induced by thermal stress. Ho et al. not only does not consider the problems encountered with high intensity illumination, but these specific problems incurred in a reflective structure, and the solutions thereof, are also not considered by Ho et al.
As another recent reference, U.S. Patent Application Publication 2003/0117708 (Kane) discloses a sealed enclosure comprising of a wire grid polarizing beamsplitter, a spatial light modulator and a projection lens having the interior space filled with a inert gas or vacuum. Among the goals stated in U.S. 2003/0117708 are protection of the wire grid component from corrosion and handling and modular packaging of the optics assembly. While this approach may be useful in some small-scale projection environments employing only a single spatial light modulator, the apparatus and method of U.S. 2003/0117708 would not be suitable for the high-heat environment of a full-color projection apparatus designed for commercial use, such as for use in motion picture theaters. Moreover, high-quality digital projection requires the use of a separate spatial light modulator for each color channel, with high-quality projection optics. In order to provide suitable contrast, additional support components for the polarizing beamsplitter are needed to provide further polarization selectivity. The relative alignment of these supporting polarization components with the polarizing beamsplitter and with the overall imaging path is significant. No provision is made for deploying or adding these supporting components in U.S. 2003/0117708. In addition, the U.S. 2003/0117708 methods do not anticipate nor provide solutions due to thermal distortion and stress birefringence that would be induced in a high-heat environment, as a result of over constraint and heat containment within the sealed enclosure.
Thus it can be seen that, while wire grid polarizers and polarizing beamsplitters offer some advantages for digital projection apparatus, problems of alignment and complexities presented by thermal expansion effects must be resolved in order to obtain suitable performance from these components.
It is an object of the present invention to provide an apparatus and technique for mounting spatial light modulator and supporting polarization components that is mechanically robust, that allows thermal expansion without degrading image quality, and that allows straightforward alignment of components in the light modulation path. With this object in mind, the present invention provides a housing for mounting a wire grid polarizing beamsplitter and a spatial light modulator in alignment with an output optical path, comprising:
(a) a front plate having an opening for admitting incident illumination provided along an illumination axis;
(b) a modulator mounting plate, spaced apart from and parallel to the front plate, for mounting the spatial light modulator in the path of the illumination axis;
(c) first and second polarizer support plates, spaced apart from each other and extending between the front plate and the modulator mounting plate; the respective facing inner surfaces of the first and second support plates providing coplanar support features for supporting the wire grid polarizing beamsplitter between the inner surfaces; and
the wire grid polarizing beamsplitter being extended between and normal to the facing inner surfaces, the surface of the wire grid polarizing beamsplitter at a fixed angle with respect to the surface of the spatial light modulator on the modulator mounting plate, the fixed angle defining an output optical axis along the output optical path.
It is a feature of the present invention that it provides a modular housing for a spatial light modulator and supporting polarization components for a single color channel.
It is an advantage of the present invention that it provides a mounting method for accurately aligning a wire grid polarizing beamsplitter relative to the optical path for modulated light. Using the apparatus and method of the present invention, no adjustment to polarizing beamsplitter position is necessary once the housing is mounted in place. Only slight adjustment for spatial light modulator positioning is necessary in any color channel.
It is a further advantage of the apparatus and method of the present invention it allows conventional optical fabrication tolerances to be used in manufacture of a precision alignment housing.
It is a further advantage of the present invention that it allows replacement of the spatial light modulator for a single color channel without necessitating re-adjustment of supporting polarization components. The complete set of modulation and polarization components for a single color channel are packaged as a unit, allowing ease of removal for serviceability.
It is yet a further advantage of the present invention that it provides a mounting arrangement for polarization components that is robust and allows for thermal expansion effects.
These and other objects, features, and advantages of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings wherein there is shown and described an illustrative embodiment of the invention.